Control of the read/write head position in a disk drive relative to track centerline on the disk is necessary to insure accuracy of disk drive read and write operations. Over the years, many types of servo systems have been devised to detect and correct the alignment between the read/write head and the track centerline. One type of disk drive servo system is known as a sampled servo system. In such a system, which is characteristically closed-loop or track-following, servo information in the form of magnetic burst patterns is recorded in one or more servo sectors on each track on the surface of the disk.
This servo information is read by the read/write head during servo operations at each servo sector and is used to generate position error signals as a function of the misalignment between the head and the disk track centerline. The position error signals are input through a microprocessor which in turn performs appropriate calculations with the position error signals and outputs servo compensation signals which control the disk drive head positioning mechanism to place the read/write heads over track centerline.
One well known type of sampled servo system involves recording groups or bursts of magnetic transitions radially displaced from each other in the servo sector. These bursts are typically designated as the "A" burst and "B" burst. Due to the radial displacement of the "A" burst relative to the "B" burst in a given servo sector, the "A" and "B" bursts are displaced on either side of the track centerline. When the head is positioned exactly over track centerline, approximately one-half of the "A" burst will be read followed by one-half of the "B" burst in a time displaced fashion. As the head moves off the centerline of a track, the amplitude of one burst decreases while the amplitude of the other burst increases depending on the direction of misalignment. In this manner, a position error signal can be derived from the relative amplitudes of the bursts by rectifying and peak detecting the readout from the head as it passes over the "A" and "B" bursts, and determining the difference in amplitude between the bursts.
A significant problem not adequately addressed by prior art "A/B" servo systems is how to eliminate the effect of magnetic distortion on the "A" and "B" bursts caused by neighboring flux transitions. One type of commercial disk drive product attempts to address this problem by recording the "A" and "B" bursts in an alternating pattern. Specifically, the prior art drive uses a sector servo approach for track following, wherein "A/B" burst patterns alternate with "B/A" burst patterns from servo sector to servo sector throughout the data tracks. The A-B burst differentials measured for each servo sector in the data track being followed are used to generate position error signals indicative of misalignment between the transducer heads in the drive and data track centerline. As a result of reversing the burst patterns from sector to sector in the drive, offsets otherwise present in the A-B burst differentials due to magnetic distortion tend to average out over the course of an entire disk revolution.
The position error signals generated by the prior art alternating servo pattern drive are not in and of themselves compensated for magnetic distortion. Rather, individual position error signals generated at each servo sector are distorted in one direction or the other relative to track centerline, in effect creating a tendency for the heads to "dither" about track centerline. As long as the servo system bandwidth is sufficiently small, the dithering tendency experienced in a disk drive employing the prior art alternating servo approach to eliminating the effects of magnetic distortion may not produce noticeable proturbations as the data transducer follows track centerline. Servo systems with larger bandwidths, however, may experience problems. In addition, electrical distortion in the servo system can introduce further offsets in the position error signal which prior art disk drives do not compensate for. It would therefore be of great benefit to sectored servo disk drives if a means for eliminating the effects of magnetic distortion on the "A" and "B" bursts can be devised.